1. Field of the Invention
The present invention relates to asphalt roofing shingles, protective granules for such shingles, and processes for making such granules and shingles.
2. Brief Description of the Prior Art
Pigment-coated mineral rocks are commonly used as color granules in roofing applications to provide aesthetic as well as protective functions to the asphalt shingles. Dark blotches or streaks sometimes appear on the surfaces of asphalt shingles, especially in warmer humid climates, because of the growth of algae and other microorganisms. The predominant species responsible is Gloeocapsa magma, a blue-green algae. Eventually, severe discoloration of the entire roof can occur.
Various methods have been used in an attempt to remedy the roofing discoloration. For example, topical treatments with organic algaecides have been used. However, such topical treatments usually have only short term effectiveness, typically one to two years. Another approach is to add algaecidel metal oxides to the color granule coatings. This approach is likely to provide longer protection, for example, as long as ten years.
Companies, including Minnesota Mining and Manufacturing (3M) and GAF Materials Corporation/ISP Mineral Products Inc., have commercialized several algaecide granules that are effective in inhibiting algae growth.
A common method used to prepare algae-resistant (AR) roofing granules generally involves two major steps. In the first step, metal oxides such as cuprous oxide and/or zinc oxide are added to a clay and alkali metal silicate mixture. The mixture in turn is used to coat crushed mineral rocks. The mixture is rendered insoluble on the rock surfaces by firing at high temperatures, such as about 500° C., to provide a ceramic coating. In the second step, the oxide-covered rocks are coated with various color pigments to form colored algae-resistant roofing granules. The algae-resistant granules, alone, or in a mixture with conventional granules, are then used in the manufacture of asphalt shingles using conventional techniques. The presence of the algae-resistant granules confers algae-resistance on the shingles.
Roofing granules typically comprise crushed and screened mineral materials, which are subsequently coated with a binder containing one or more coloring pigments, such as suitable metal oxides. The binder can be a soluble alkaline silicate that is subsequently insolubilized by heat or by chemical reaction, such as by reaction between an acidic material and the alkaline silicate, resulting in an insoluble colored coating on the mineral particles.
U.S. Pat. No. 3,507,676 discloses roofing granules containing zinc, zinc oxide, or zinc sulfide, as an algaecide and fungicide.
Algae resistant shingles are disclosed, for example, in U.S. Pat. No. 5,356,664 assigned to Minnesota Mining and Manufacturing Co., which discloses the use of a blend of algae-resistant granules and non-algae-resistant granules. The algae-resistant granules have an inner ceramic coating comprising cuprous oxide, and an outer seal coating initially devoid of copper.
Algae resistant roofing granules are also disclosed in U.S. Patent Publications 2004-0258835 A1, 2004-0255548 A1, 2007-0148342 A1, 2007-0148340 A1, and 2008-0118640 A1.
Nanoparticles are known to have interesting properties due to their small particle size and their large surface area and, in some cases, high aspect ratios. For instance, nano titanium dioxide and nano zinc oxide are widely used as UV blockers or for self-cleaning surfaces due to their photocatalytic effect; and nano-silver particles have been used as biocides in textiles, medical devices, etc. However, to incorporate the nanoparticles in an inorganic coating matrix has been a challenge, particularly when considering dispersing and handling such particles of nano-size. In addition, nanoparticles can easily become airborne and easily absorbed by the operator handling the nanoparticles, thus posing potentially serious but currently unknown health or safety issues.
Nanoparticles that are pre-dispersed in liquid medium are commercially available. However, they may not be suitable for some coating applications due to the presence of organic dispersants. Also, such pre-dispersed nanoparticles may require additional processing at a higher cost for use in some coatings applications. One approach to associate nanoparticles into micro-size particles is described in European patent application EP 1889810 A1 and in U.S. Patent Publication 2009/0131517 A1, which disclose dispersing nanoparticles of silver and/or copper and/or their oxides in silica using a flame spray pyrolysis technique to provide antibacterial and antifungal materials.
There is a continuing need for algae-resistant roofing products having algaecide leaching rates that can be controlled so that the roofing products can be tailored for specific local conditions.